While rummaging through a bookstore, (Yes, they still exist), I happened upon this alluring title: Why We Get Sick, with the equally intriguing subtitle of The New Science of Darwinian Medicine. The book, written by a Professor of Psychiatry at Michigan and an Emeritus Professor of Ecology and Evolution at Stony Brook, looks at disease through the lens of the evolutionary biologist.
The focus is not the traditional one of disease, symptoms, and its treatment, but rather what causes diseases to exist at all. Why hasn't Natural Selection perfected a human body that is impervious to disease as well as endowed with repair capacities to resist the aging process?
The authors note the paradox of a human body with its extraordinary complexity and precision burdened with striking examples of flaws and frailties-all due to the process of Darwinian natural selection. In the authors' view, natural selection results in a bundle of compromises. It optimizes the survival of genes and reproduction of the species, but not necessarily in ways that optimize health or benefit the survival of the post-reproductive adult.
As noted in the book, diseases have six evolutional explanations:
1. The body's own defense mechanisms which produce symptoms;
2. Infectious disease as an evolutionary arms race between rapidly evolving intruders and the multiple resistance mechanisms of the more slowly evolving host;
3. Novel environmental changes between present living conditions and those of our hunter-gatherer forbearers;
4. Genetics, with the constant mutation rates in a complex multi-celled organism like man;
5. Design compromises, such as our upright stance that leads to chronic back problems; and
6. Evolutionary legacies that result from the evolutionary dependence on incremental changes with the inability to reboot and start anew.
For each of these evolutionary contributions to disease, the authors provide at least one and often several examples. Defense mechanisms for ridding the body of toxins or infections such as cough, vomiting, and diarrhea may be beneficial to the host, but paradoxically may serve to spread the disease to others. Infectious diseases pit rapidly evolving infectious agents against the body's resistance as well as the science of antibiotic development.
Many human frailties such as high-fat diets and sweets may have been beneficial to our Stone Age ancestors but not to us. Genetic diseases such as sickle cell trait benefit those in Malaria-prone areas while compromising the bearer in other ways. Design compromises such as the shared entry of the respiratory and alimentary tracts produce their own self-inflicted difficulties. The authors maintain that our air-food traffic problem started with a remote ancestor that didn't need respiration, but rather lived by passive oxygen diffusion.
When respiration was required, it was simply "bolted on" the preexisting system. Even Darwin in 1859 noted "the strange fact that every particle of food and drink has to pass over the orifice of the trachea with some risk of falling into the lungs, notwithstanding the beautiful contrivance by which the glottis is closed." Natural selection avoids overdesign and if something works well enough, that there is no evolutionary mechanism for complete redesign.
To their credit, the authors admit that most of their evolutionary explanations are just hypotheses and most lack proof. They feel that there has been too little research done on Darwinian Medicine and that no funding source is readily available for such work. That said, I found that most of the examples provided in the text would be familiar to many physicians and that most doctors do consider evolutionary contributions to disease.
Nesse and Williams devote an entire chapter to cancer. Even with mutation rates of one in 10,000, complex organisms like man with ten trillion cells would have almost a billion mutations, potentially initiating cancerous growth throughout the body. Complex defense mechanisms such as tumor-suppressor genes, immune systems, and apoptosis continually reduce risk.
As the body ages, senescence results in reduced vigor of those defenses along with increased and prolonged exposure to toxins and environmental carcinogens. From an evolutionary perspective, it is not difficult to see cancer as a disease of aging. To add further to the problem, there is no Darwinian advantage to select for protective mechanisms to aid the post-reproductive age adult.
One difficulty with the book, first published in 1995, is that much of the medical information is now out of date. While 18 years is a drop in the evolutionary bucket, medical advances move rapidly. For example, we now know that humans have only 20,000 genes rather than the 100,000 mentioned in the book. The resistance problems with AZT therapy have long since been corrected by highly active antiretroviral therapy (HAART).
The authors view medicine as a practical enterprise but say that we should also consider evolutionary explanations for disease. Medicine, they believe, has not taken up the evolutionary aspect of disease because it is not immediately obvious how evolutionary explanations might help prevent or treat disease.
Still, in spite of examples like this, this reader finds those criticisms justified. Applying Darwin's ideas does help us understand why we are susceptible to disease, but for the most part, doesn't help us reduce our fat intake or solve our back problems by walking on all fours.
That said, the authors do an admirable job of illustrating the human body's evolutionary compromises. Despite our exquisite design, our bodies have crude flaws. Despite multiple defenses we have a thousand vulnerabilities. Despite repairs, our bodies deteriorate and ultimately fail. Those flaws result from evolutionary pressures that favor selfish genes and survival of the species at the price of susceptibility to disease.
Although there are shortcomings to the book, the authors do make us step back from our usual practical focus on treating illness to look at Why We Get Sick from an evolutionary perspective.
1995, KNOPF DOUBLEDAY PUBLISHING GROUP, ISBN 0679746749
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